The present invention relates to a manual aperture value setting circuit for an interchangeable lens camera.
An aperture-priority system whereby an aperture value is set by the photographer and the shutter speed is then automatically set in accordance with the chosen aperture value, the brightness of the scene being photographed and the film sensitivity has been well known for use in an interchangeable lens camera. The following aperture value setting methods in the aperture-priority system are available:
(1) First Method: A relay lever in the camera body is operated by turning the aperture ring of a lens mounted on the camera, and a resistance of a variable resistor which responds to the operation of the relay lever is electrically detected so that the set aperture value is electrically read by a circuit in the camera body.
(2) Second Method: An index mark such as "A" (representing "Automatic") is provided on the aperture ring of the lens, the operation of the aperture is controlled on the side of the camera body when the aperture ring is set to the index mark "A", and electrical data such as data in Gray code form is inputted to an arithmetic circuit by turning an aperture value disk which is graduated with aperture values.
The present invention relates specifically to an improvement of the second method of manually setting an aperture value on the side of the camera body.
In the second method, an aperture value setting member, namely, the aperture value disk, is of a mechanical construction. Therefore, the second method suffers from difficulties that it is considerably difficult to limit the range of operation of the aperture value disk (hereinafter referred to as "an aperture value range" when applicable) to a range of aperture values from the full-aperture value to the minimum aperture value of a lens mounted on the camera, and if the shutter is released with an aperture value set out of the aperture value range by mistake, exposure is carried out incorrectly. An aperture value set on the side of the camera body may be within the aperture value range of the camera until the lens is replaced by another lens. However, it may occur that, when the lens is replaced by another lens of different aperture value range, the aperture value which has been previously set is out of the aperture value range of the lens which has been newly mounted. Thus, in this case also, the same difficulty occurs.